This application relates to co-pending U.S. patent application Ser. No. 09/470,359 entitled xe2x80x9cAir-Fuel Charge Controller For A Homogeneous-Charge, Compression-Ignition Engine,xe2x80x9d filed Dec. 22, 1999, and U.S. patent application Ser. No. 09/573,743 filed on May 18, 2000, entitled xe2x80x9cCycle Strategies For A Hybrid HCCI Engine Using Variable Camshaft Timing.xe2x80x9d These applications are commonly owned by the assignee of the present invention.
This invention relates to an engine control strategy for a hybrid homogeneous-charge compression-ignition (HCCI) and spark ignition (SI) engine, and more particularly to a strategy for combustion mode transition between HCCI and SI engine operation.
The HCCI engine is a relatively new type of engine. It has the benefits of very low NOx emissions due to the low combustion temperatures of the diluted mixture, and zero soot emissions due to the premixed lean mixture. Also, thermal efficiency of the HCCI engine is higher than SI engines and is comparable to conventional compression ignition engines due to the high compression ratio, unthrottled operation, high air-fuel ratio, reduced radiation heat transfer loss, and the low cycle-to-cycle variation of HCCI combustion.
The limitations of HCCI engines of known design relate to controlling the ignition timing and the combustion rate at different operating conditions. This is because combustion starts by auto-ignition when the mixture reaches a certain temperature. Thus, the air-fuel mixture is formed earlier before top dead center (TDC), and ignition can occur at any time during the compression process. Thus, as the engine load increases, the ignition tends to advance, and the combustion rate tends to increase due to the richer mixture. The thermal efficiency may also decrease due to the early heat release before TDC, and the engine becomes rough due to fast and early combustion.
When the engine load decreases, the ignition tends to be retarded which may eventually result in misfiring as well as an increase in HC and CO emissions. When the engine speed increases, the time for the main heat release tends to be retarded since the time available for low-temperature preliminary reaction of the diluted mixture becomes insufficient and misfiring may occur.
Supercharging the intake with a large amount of exhaust gas recirculation (EGR) can extend the HCCI operating range to higher loads. This approach, however, is constrained by peak cylinder pressure which becomes higher at higher intake pressure. In addition, during engine cold start, HCCI combustion is almost impossible unless a very high compression ratio is applied. For these reasons, application of HCCI technology for an automotive engine which requires a wide range of operating conditions, requires a dual HCCI-SI combustion mode engine. The engine operating conditions for HCCI combustion and for SI combustion, however, are quite different. To switch the combustion mode of the engine between HCCI and SI, the engine control parameters must be changed precisely and quickly. Otherwise, the engine may produce noticeable torque changes.
An object of the present invention is to provide a control system and method for operating a gasoline powered hybrid HCCI-SI engine over a wide load range including cold start. It is another object of the invention to provide a smooth transition between HCCI and SI combustion modes.
According to the present invention, the foregoing and other objects are attained by a method of operating a hybrid homogenous-charge compression-ignition (HCCI) engine and spark ignition (SI) engine. The engine includes at least two cylinder groups, and each of the cylinder groups further include at least one cylinder. The method comprises the steps of detecting a transition request to transition the engine from a current operating mode to a desired operating mode. For example, a transition request from HCCI combustion mode to SI combustion mode, or vice versa. In response, the method transitions the first cylinder group from the current engine operating mode to the desired engine operating mode during a first time period, and transitions the second cylinder group from the current engine operating mode to the desired engine operating mode during a second time period. The second time period begins after the start of the first time period. In one aspect of the invention, the second time period begins after the end of the first time period, and in another aspect of the invention, the second time period overlaps the first time period.
An advantage of the present invention is that it smoothly transitions between HCCI and SI combustion modes. Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.